The Meteoric Isotope Line

Harmon Craig published (1961a) a dD and d18O diagram, based on about 400 water samples of rivers, lakes, and precipitation from various countries (Fig. 9.4). An impressive lining of the data along the best-fit line of dD = d18O + 10

has been obtained. Outside this line plot data from East African lakes that undergo significant isotopic fractionation due to intensive evaporation losses.

Fig. 9.4 Isotopic data of about 400 samples of rivers, lakes, and precipitation from various parts of the world. The best-fit line was termed the meteoric line. Its equation, as found by Craig (1961a), is dD = 8d18O + 10. The data in the encircled zone of ''closed basins'' is for East African lakes with intensive evaporation.

Fig. 9.4 Isotopic data of about 400 samples of rivers, lakes, and precipitation from various parts of the world. The best-fit line was termed the meteoric line. Its equation, as found by Craig (1961a), is dD = 8d18O + 10. The data in the encircled zone of ''closed basins'' is for East African lakes with intensive evaporation.

The data in Fig. 9.4 lie on a straight line in spite of the very wide range of values: dD of -300% to +50%, and d18O of -42% to +6%. This line, called the meteoric line, has been found, with some local variations, to be valid over large parts of the world.

The meteoric line is a convenient reference line for the understanding and tracing of local groundwater origins and movements. Hence, in each hydrochemical investigation the local meteoric line has to be established from samples of individual rain events or monthly means of precipitation. A specific example of a local meteoric line, from northeastern Brazil, is given in Fig. 9.5. A local meteoric line is obtained: dD = 6.4 d18O + 5.5 (Salati et al., 1980). Examples of equations of local meteoric lines reported from various parts of the world are given in Table 9.1.

The composition of precipitation is reflected, directly or modified, in the composition of groundwater. Common practice is to plot groundwater data on dD — d18O diagrams, along with the meteoric line of local precipitation as a reference line. Examples are given in Figs. 9.6-9.8. In Fig. 9.6 the composition of the groundwater plots are close to the local meteoric line, ruling out secondary processes, such as evaporation prior to infiltration or isotope exchange with aquifer rocks.

In Fig. 9.7 the groundwater data fall distinctly below the relevant meteoric line, indicating that secondary fractionation has occurred, or that the waters are ancient and were recharged in a different climatic regime that

Fig. 9.5 Isotopic composition of precipitation in the Pajeu River basin, Brazil: O, months with rain over 50 mm/month; •, months with lower precipitation amounts. A local meteoric line is obtained with the equation dD = 6.4d18O + 5.5. (From Salati et al., 1980.)

Table 9.1 Examples of Regional Meteoric Lines

Region

Meteoric line (%)

Reference

''Global'' meteoric line Northern hemisphere, continental Mediterranean or Middle East

Maritime Alps (April 1976)

Maritime Alps (October 1976)

Northeastern Brazil Northern Chile Tropical islands dD = 8d O + 10 dD = (8.1 ± 1)d18O + (11±1)

dD = (7.9±0.2)d180 + (13.4 + 2.6) dD = 6.4d180 + 5.5 dD = 7.9d180 + 9.5 dD = (4.6±0.4)d180 + (0.1 + 1.6)

Craig (1961a) Dansgaard (1964)

Gat (1971)

Bortolami et al. (1978)

Bortolami et al. (1978)

Salati et al. (1980) Fritz et al. (1979) Dansgaard (1964)

was characterized by a different local meteoric line. What should be checked in order to decide between these two possible explanations? The age. Groundwater data plotted in Fig. 9.8 fall along the meteoric line but reveal a large spread of values. Separation into shallow and deep waters revealed the

Fig. 9.6 Isotopic composition of water sampled from wells in central Manitoba, Canada. The values fall close to the local meteoric line (dD = 8.1d18O + 11). The researchers (Fritz et al., 1974) concluded that evaporation during recharge and isotopic exchange with aquifer rocks are insignificant.

Fig. 9.7 Isotopic composition of groundwaters of northern Chile. The values lie below the meteoric line of local precipitation, explained by the investigators (Fritz, et al., 1979) as reflecting secondary fractionation by evaporation prior to infiltration, or the presence of ancient waters that originated in a different climatic regime. The large variations in the groundwater compositions are useful in local groundwater tracing.

Fig. 9.7 Isotopic composition of groundwaters of northern Chile. The values lie below the meteoric line of local precipitation, explained by the investigators (Fritz, et al., 1979) as reflecting secondary fractionation by evaporation prior to infiltration, or the presence of ancient waters that originated in a different climatic regime. The large variations in the groundwater compositions are useful in local groundwater tracing.

Fig. 9.8 Isotopic composition of groundwaters near Chatt-el-Honda, Algeria. The values scatter along the meteoric line but reveal an internal order: values of deep groundwaters are isotopically lighter (more negative) than shallow groundwaters. This was taken as an indication that the deep groundwaters were ancient and originated from rains of a different climatic regime. This is supported by the large number of ancient deep groundwater water samples, as borne out by low 14C concentrations (sections 11.4 and 11.8). (From Gonfiantini et al., 1974.)

Fig. 9.8 Isotopic composition of groundwaters near Chatt-el-Honda, Algeria. The values scatter along the meteoric line but reveal an internal order: values of deep groundwaters are isotopically lighter (more negative) than shallow groundwaters. This was taken as an indication that the deep groundwaters were ancient and originated from rains of a different climatic regime. This is supported by the large number of ancient deep groundwater water samples, as borne out by low 14C concentrations (sections 11.4 and 11.8). (From Gonfiantini et al., 1974.)

latter have distinctly lighter isotopic compositions. A possible origin of ancient recharge at a different climatic regime was suggested in this case too. The necessary check has been done: the deeper waters, of light isotopic composition, were indeed found to contain little or no measurable 14C, indicating high groundwater ages (section 11.4) and supporting the paleoclimate hypothesis.

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